Alhena (Gamma Geminorum)
A subgiant A star and the third
brightest member of the constellation Gemini.
Its Arabic name (alternatively given as Almeisan) refers to a brand on a
horse or a camel. Alhena is a spectroscopic
binary with a period of 12.6 years and is the brightest star ever observed
to be occulted by an asteroid. In 1991, (381)
Myrrha passed in front of Alhena enabling not only the asteroid's diameter
(140 km) to be determined but also the fact that the dimmer companion star
is a Sun-like G star almost 200 times fainter
than Alhena proper. Accumulated observations have shown that the companion,
of about one solar mass, orbits the 2.8-solar-mass primary at an average
separation of about 8.5 AU – about the size of Saturn's
orbit – but ranges from about as close as Earth is to the Sun to about
the distance of Uranus.
|Alhena (center) with Mu Geminorum at upper right.
Image: © Akira Fujii
||105 light-years (32 pc)
||R.A. 06h 37m 42.7s,
Dec. +16° 23' 57"
||Almeisan, 24 Geminorum,
HR 2421, BD +16°1223,
HD 47105, GCTP1539.00,
SAO 95912, FK5 251, HIP 31681